There are a number of types of piezoelectric devices that have been designed to perform electronic signal processing or to measure such variables as mass, pressure, viscosity and density. For example, a gravimetric sensor may be used to measure the concentration of a selected class of compounds in a chemical solution into which the sensor is immersed. In addition to use with liquids, piezoelectric sensors may be utilized with gases.
As used herein, such piezoelectric devices are broadly classified as "bulk wave devices," "plate wave devices" and "surface wave devices." A bulk wave device is one in which an acoustic wave tends to propagate and extend throughout the full thickness of a piezoelectric substrate. A plate wave device is one in which acoustic energy is confined by reflection from the top and bottom surfaces of a plate. A surface wave device is one in which acoustic energy is confined in a vertical direction (i.e. a direction perpendicular to a substrate surface) in a region adjacent to the substrate surface.
Each of the three classes can be subclassified by the orientation of the acoustic wave motion with regard to the substrate surface of the device. The three types of wave motion are: (1) longitudinal wave motion in which material displacement is in a direction parallel to the direction of propagation of the wave; (2) shear vertical wave motion in which material displacement is in a direction perpendicular to both the substrate surface and the direction of wave propagation; and (3) shear transverse, or shear horizontal, wave motion in which material displacement is perpendicular to the direction of propagation and parallel to the substrate surface.
A "Surface Acoustic Wave" (SAW) device is one type of surface wave device. This type is also known as a "Rayleigh Wave" (RW) device and utilizes waves that are predominantly shear vertical, with the energy localized within an acoustic wave length of the substrate surface. While this type operates efficiently in many applications, the shear vertical wave motion may adversely affect performance when the SAW device is used as a sensor in a liquid. The shear vertical component of wave motion presses against the fluid under test. If the surface wave velocity is greater than the fluid compressional wave velocity, energy will be radiated into the fluid. Because the energy of the surface wave leaks away into the fluid, the fluid compressional waves are called "leaky waves." The attenuation resulting from leaky-wave radiation into a fluid causes an unacceptable amount of insertion loss, rendering the device inoperable for such use.
A "Flexural Plate Wave" (FPW) device also utilizes shear vertical wave motion. An FPW sensor has a thin plate that is fabricated by using conventional semiconductor fabrication techniques. An FPW chemical sensor is described by Wenzel et al. in "Flexural Plate-wave Gravimetric Chemical Sensor," Sensors and Actuators, A21-A23 (1990), pages 700-703. A region for the flow of a vapor or a liquid is etched into a silicon substrate and an ultrasonic delay line consisting of a composite plate of low-stress silicon nitride, aluminum and zinc oxide is used as the plate for the top-to-bottom reflection of wave energy. The FPW sensor has the advantage of exhibiting an acoustic velocity that is less than the acoustic velocity of most liquids, thereby avoiding the "leaking away" of wave energy into the liquid. However, the FPW sensor is overly sensitive to small changes in liquid density, pressure and temperature. Moreover, the sensor is relatively fragile, since the plate is extremely thin, as necessitated by a low acoustic velocity.
Shear transverse wave motion is the preferred orientation of acoustic wave motion within a fluid sensor. Shear transverse waves are not affected by the same leaky wave mechanism, since the material displacement at the fluid/substrate interface is parallel to the substrate surface and does not press against the fluid. The absence of the surface-normal component of material displacement allows the shear transverse waves to propagate without unacceptable amounts of wave energy dissipation into the fluid under test.
An "Acoustic Plate Mode" (APM) liquid sensor using shear transverse waves is described by Martin et al. in "Characterization of SH Acoustic Plate Mode Liquid Sensors," Sensors and Actuators, 20 (1989), pages 253-268. Martin et al. teach use of a thin, single-crystalline quartz plate which acts as an acoustic wave guide to confine acoustic energy between upper and lower surfaces of a plate as the waves propagate from an input to an output transducer. The upper and lower surfaces of the quartz plate impose a transverse resonance condition, such that the APM has displacement maxima at the surfaces, with sinusoidal variation between the surfaces. Because of the characteristics of plate wave devices, sensing can take place on a side of the quartz plate opposite to the transducers. APM sensors are less susceptible to leaky-wave attenuation, but are typically less sensitive than SAW devices.
A "Surface Skimming Bulk Wave" (SSBW) device utilizes shear transverse wave motion. The Surface Skimming Bulk Wave type is also referred to as a "Shallow Bulk Acoustic Wave" (SBAW) device. Propagation occurs by way of bulk mode, in which the waves graze the surface and diffract into the piezoeleotric substrate. Bulk propagating modes have higher velocities than Rayleigh waves, but are more susceptible to losses due to inefficient coupling of power to and from the substrate. Moreover, diffraction losses are significant.
"Love Wave" (LW) devices differ from an SSBW by the inclusion of a plate that functions as a surface trapping structure to trap the wave energy proximate to the surface of the piezoelectric substrate. Addition of the plate provides mass loading and causes piezoelectric shorting which slows down the skimming bulk shear wave, thereby creating a decay of the wave function into the depth of the substrate. The material selected in fabricating the plate is conventionally one having a lower acoustic shear wave velocity than the piezoelectric substrate, so that the plate slows the shear transverse wave even further.
A "Surface Transverse Wave" (STW) device also utilizes shear horizontal wave motion. The STW device differs from the Love Wave device only by the replacement of the wave-trapping plate with surface grooves or with a raised grating of fingers. The grating of fingers provides stronger surface trapping than the plate. Thus, high velocity bulk modes are further trapped near the surface of the substrate, allowing an even greater coupling of power through more efficient transduction.
Typically, an LW sensor or an STW sensor includes an input interdigital transducer having an array of interleaved electrode fingers to launch shear transverse waves along a sensing region of a piezoelectric substrate in response to an electrical signal. On the opposite end of the sensing region is an output interdigital transducer, which detects the waves and generates a corresponding output signal. In its simplest form, such sensors act as highly sensitive detectors of changes in surface mass, responding to accumulated mass per unit area. More sophistication is achieved by coating the surface of the piezoelectric substrate with a chemically reactive layer that preferentially reacts with a constituent within the fluid under test. Depending upon the concentration of the constituent within the fluid, the mass of the chemically reactive layer will fluctuate. The change in mass of the layer causes a corresponding change in the phase delay or acoustic shear wave velocity of the sensor. Thus, the sensor may be dedicated to detection of a specific constituent, such as a particular antibody within a solution.
One concern in the use of an LW sensor or an STW sensor is the effect of the fluid under test on the input and output interdigital transducers. Typically, the electrode fingers of each of the transducers are interleaved metallic members. Depending upon the fluid under test, the fluid may cause corrosion of the electrode fingers. Moreover, the fluid may electrically short the electrode fingers together. Therefore, preferably the fluid is sealed within the sensing region of the sensor and prevented from reaching the interdigital transducers. For example, a flow cell may be mounted to the surface of the piezoelectric substrate and a compliant gasket may be sandwiched between the flow cell and the substrate surface.
Sealing the fluid flow from the interdigital transducers solves the problems of electrical shorting and premature corrosion, but creates other problems. Firstly, the compliant gasket is another source of leaky-wave attenuation. Wave energy leaks away from the sensor substrate into the compliant gasket in the form of shear waves. Secondly, in addition to leaky-wave attenuation, other mechanisms cause the gasket to reflect or absorb wave energy, leaving a smaller fraction of wave energy propagating from the input interdigital transducer to the output interdigital transducer. The wave attenuation increases with the length and the mechanical rigidity of the gasket or other sealing member. Consequently, the choice of the means for providing a fluid-tight seal represents a compromise between attenuation and fluid sealing considerations. That is, the compromise is between the sensitivity of the sensor and the reliability of the seal.
It is an object of the present invention to improve the sensitivity and performance of Surface Transverse Wave devices and Love Wave devices that are utilized for fluid sensing, wherein the improvement is achieved without compromising the reliability of a fluid-tight seal for containment of a fluid under test.